Method for reducing copper solubility at the inner surface of a copper tube

ABSTRACT

In a method for reducing copper solubility at the inner surface of a copper tube, the process parameters surface treatment (degreasing and pickling), flow conditions (flow speed&lt;1 m/s), temperature (50° C. to 80° C.) and time (1 min to 10 min) are purposefully adjusted to one another, so as to achieve a uniformly directed crystal growth during the course of the tin coating. In particular, the planes (101) of the copper crystals and the tin crystals are aligned parallel to one another and the directions [101] are aligned perpendicular to one another.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method for reducing copper solubilityat the inner surface of a copper tube.

[0003] 2. Description of Related Art

[0004] It is known that one can coat the inner surface of a copper tubewith tin, in order to prevent, with the aid of such a tin layer, copperions dissolved from the copper from going over into drinking water, ifsuch a copper tube is being used as a component of a drinking waterline. In this connection, the European guideline for drinking waterregulations should be noted.

[0005] In the methods known up to this point for depositing a tin layeron the inner surface of a copper tube, only random crystals are formed.The packing density of the tin crystals was therefore unsatisfactory.Thus, copper ions are able to go over into the drinking water by way ofthe tin layer.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to create a method that willensure a clear reduction of copper solubility at the inner surface ofthe copper tube.

[0007] These and other objects of the invention are achieved by a methodfor reducing copper solubility at the inner surface of a copper tube inwhich the process parameters surface treatment (degreasing andpickling), flow conditions (flow speed<1 m/s), temperature (50° C. to80° C.) and time (1 min to 10 min) are purposefully adjusted to oneanother, so as to achieve a uniformly directed crystal growth during thecourse of the tin coating and the formation of a copper/tin phase.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The invention recognized that a high packing density of tincrystals is able to be reached in the case of a directed crystal growthduring the course of the tin coating. The high packing density of thetin layer leads to a very homogeneous, evenly formed and most stablecopper/tin phase. In addition to that, the tin layer is deposited evenlyand having a thickness of an order of magnitude of about 0.1 μm to 3 μm,at a low pore number. The directed crystal growth is achieved bypurposeful setting of the process parameters surface pretreatment(degreasing and pickling), flow conditions (flow speed<1 m/s),temperature (50° C. to 80° C.) and time (1 min to 10 min).

[0009] In spite of the fact that, even after a relatively shortoperating time, metallic tin can no longer be detected at the tubesurface, still, the property of the low copper solubility because of theextremely stable Cu/Sn phase is maintained. Tube surfaces treatedaccording to the invention distinguish themselves by having a very lowcopper solubility and a high stability.

[0010] Ascertaining the crystal structure of tin is preferably doneusing the results of X-ray diffraction experiments. During theinteraction of the X-rays and the monocrystals, the incident waves arediffracted in discrete spatial directions on account of the lattice-likestructure of the tin crystals. Whereas the position of a diffractiondirection is determined by the orientation of the crystal lattice to theprimary beam, the lattice dimensions and the wavelength used, theintensity of the diffracted beam depends on the distribution of the tinatoms in the unit cell.

[0011] For this purpose, a length section was cut off as a sample fromthe copper tube coated with tin according to the invention. This samplewas then slit in the longitudinal direction, and the longitudinally slitsample was subsequently bent open to form a flat material. This flatmaterial with the tin coating facing upwards was then irradiated withX-rays having lambda(FeK_(a))=1.9373 Å, the X-ray being directed at theflat sample at various diffraction angles. The results show that, atvarious angles, a high X-ray diffraction intensity was detected, whichproves that the X-rays were reflected close to 100%. It follows that thetin atoms have a very high packing density. A satisfactory barrier layeris formed for the copper ions against transition into the drinkingwater.

[0012] In an advantageous further design of the basic idea according tothe invention, plane (101) of the tin crystal is aligned parallel to theinner surface of the copper tube.

[0013] In this connection, it is then further advantageous, if plane(101) of the tin crystals is aligned parallel to plane (101) of thecopper crystals.

[0014] A further improvement going beyond this is achieved, if planes(101) of the copper and tin crystals are aligned parallel to each otherand directions [101]are aligned perpendicular to each other.

[0015] The advantageous tin coating was confirmed by the followinginternal comparison investigation:

[0016] A copper tube coated according to the invention was brought intocontact with drinking water for 15 months. As a result of this contact,a tin hydroxide layer (Sn₃O₂(OH)₂) was formed on the inner surface. Fromthis copper tube a lengthwise section was removed, cut open in thelongitudinal direction, bent flat, and then submitted to an X-rayirradiation having lambda(FeK_(a))=1.9373 Å. It was thereby determinedthat the reflection of the X-rays (X-ray diffraction intensity) wasclearly less than 100%. Thus, one might have assumed that the object ofthe present invention was not attained.

[0017] The tin hydroxide layer was subsequently completely removed, sothat the original coating state was present again. That is, on thecopper there was a copper/tin phase (Cu₆Sn₅).

[0018] Then an X-ray irradiation was again carries out, at which time analmost 100% reflection was found. Thereby the method according to theinvention was fully confirmed in its advantageousness.

What is claimed is:
 1. A method for reducing copper solubility at theinner surface of a copper tube, comprising adjusting: surface treatment;flow conditions, including flow speed<1 m/s; temperature between 50° C.and 80° C., and time between 1 min and 10 min with respect to oneanother, so as to achieve a uniformly directed crystal growth during thecourse of a tin coating and formation of a copper/tin phase.
 2. Themethod according to claim 1, wherein the surface treatment adjustedincludes degreasing and pickling.
 3. The method according to claim 1, inwhich the plane (101) of the tin crystals is aligned parallel to theinner surface of the copper tube.
 4. The method according to claim 2, inwhich the plane (101) of the tin crystals is aligned parallel to theinner surface of the copper tube.
 5. The method according to claim 1, inwhich the plane (101) of the tin crystals is aligned parallel to theplane (101) of the copper crystals.
 6. The method according to claim 2,in which the plane (101) of the tin crystals is aligned parallel to theplane (101) of the copper crystals.
 7. The method according to claim 3,in which the plane (101) of the tin crystals is aligned parallel to theplane (101) of the copper crystals.
 8. The method according to claim 4,in which the plane (101) of the tin crystals is aligned parallel to theplane (101) of the copper crystals.
 9. The method according to claim 1,in which the planes (101) of the copper crystals and the tin crystalsare aligned parallel to each other and directions [101] are alignedperpendicular to each other.
 10. The method according to claim 2, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 11. The method according to claim 3, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 12. The method according to claim 4, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 13. The method according to claim 5, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 14. The method according to claim 6, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 15. The method according to claim 7, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.
 16. The method according to claim 8, inwhich the planes (101) of the copper crystals and the tin crystals arealigned parallel to each other and directions [101] are alignedperpendicular to each other.